The present invention is directed toward dynamic compressors. More specifically, the present invention is directed toward a method utilized by a control system within a dynamic compressor in order to provide surge protection for the dynamic compressor.
A typical dynamic compressor has a gas inlet and a gas outlet wherein the compressor is driven by a compressor driver so that the gas, while flowing through the compressor, is compressed. A problem associated with dynamic compressors is the amount of gas flow that is provided at the gas inlet. Specifically, if an insufficient amount of gas flow is provided, a surge within the system occurs causing damage to the compressor. Because of the high price of compressors great care must be taken to ensure that compressors are not damaged.
To minimize damage to compressors as a result of lack of gas flow at the inlet, a recycle or surge control valve is installed around dynamic compressors where the surge control valve opens to divert gas flow from the outlet of the compressor and recycle it to the inlet of the compressor to ensure that there is always sufficient gas flow in the inlet to prevent surges from occurring.
As a result of the need to protect against surge, control systems have been provided to control the operation of the surge control valve. Compressor surge control systems (also known as anti-surge controllers) use a standard PID (Proportional, Integral, Derivative controller for regulating the surge control valve when flow rate decreases below a predefined set point. The set point for recycling is established based on heuristic rules and operating guidelines typically set as a fixed margin from a surge limit line (or minimum flow set point).
To improve upon this system, some control systems in the prior art employ the use of a fixed and variable set point to adjust the safety margin based on compressor flow rate changes. The problem with these control systems is that the rate of approach to surge is determined based on the derivative of a flow signal that is typically very noisy in field installations. The control system dampens out the noisy signal through the use of passive digital filters rendering the variable set point determination ineffective and impractical in the field. In addition, these control systems are very difficult to tune in the field because these techniques are not self adapting to varying process conditions.
As a result of the above problems one solution has been to present a second set point established from the surge limit line that is considered a minimum set point for recycling and is sometimes referred to as a fast open control line. The fast open control line acts as a safeguard defense to open the surge control valve further and in a quicker manner in order to protect the compressor. Problems exist with this system because again, the set point is determined in a heuristic fashion. Typically, in order to initiate the quick response, control systems use open loop methods or alternatively closed loop variable gain methods to increase the controller gain to initiate quick response. Alternatively, some controllers employ the use of derivative based open loop valve jumps to quickly open the recycle valve when the compressor operating point crosses the fast open control line.
The problem with open loop control response is that an open loop control response is difficult to set up in the field. Moreover, with the degradation of the control elements such as valves or non linearity in process dynamics with changing plant demand, the open loop control parameters need to be retested frequently to match the operating process characteristics. This renders the compressor system performance less than optimum. A direct result of this is decrease in process efficiency, process instability, and even compressor damage from surge and process shutdown.
For control systems with closed loop fast response using variable gain method, it is very difficult to establish the control loop tuning in the field. In addition, this method can make the control system ineffective or even unstable.
For control systems that use derivative based open loop control response the challenges come from noisy flow signals used to characterize response. In these systems the control system dampens out noisy signals through the use of passive digital filters thus rendering the open loop derivative response determination ineffective and impractical to tune in the field.
Control systems based on closed loop rate limiting of the surge variable such as U.S. Pat. No. 5,798,941 suffer from the aspect of using a noisy derivative of flow signal as a process variable to determine quick valve opening. In addition, another problem is that establishing the rate set point for optimum control response is very difficult for a field engineer. Further, tuning a prior art control system in the field also is extremely challenging because the known prior art techniques are not self adapting to changes in field conditions and instead require high level of expertise in the field.
Thus, a principal object of the present invention is to provide a control system and method for providing improved protection for a dynamic compressor.
Yet another object of the present invention is to provide an adaptive safety margin determination based on process disturbance modeling using a compressor load variable.
Another object of the present invention is to provide an adaptive surge preventer control system for a turbo compressor.
These and other objects, features and advantages will become apparent from the specification and claims.